This invention relates to medical diagnostic ultrasonic imaging methods and systems, and in particular to improvements to such systems that allow an increased frame rate.
In various medical diagnostic ultrasonic imaging applications, multiple transmit beams are fired along the same ultrasound line. Examples of such applications include two-pulse techniques that use phase inversion subtraction to enhance harmonic image components, synthetic aperture techniques, synthetic spectrum techniques, and sequential focus techniques. The requirement for multiple transmit pulse firings on each ultrasound line results in a substantial reduction in frame rate.
For example, Chapman U.S. Pat. No. 5,632,277 discloses an ultrasound imaging system that employs phase inversion subtraction to enhance the image. In the disclosed system, two transmit ultrasonic pulses which differ in phase by 180° are focused in the same beam direction. The echo signals associated with these pulses are stored and then summed. Linear echoes destructively interfere in this summation due to the 180° phase difference between the two transmitted ultrasonic pulses. Non-linear echoes do not destructively interfere to the same extent, because the phases associated with non-linear echoes no longer differ by 180°. In this way, the amplitude of the non-linear echoes can be increased relative to the amplitude of the linear echoes in the summed signal.
The system disclosed in the Chapman patent suffers from the disadvantage that two ultrasonic pulses must be directed along each beam direction, and this requirement reduces the frame rate by a factor of two.
Similarly, Cole U.S. Pat. No. 5,617,862 discusses a system that coherently sums receive beams along the same steering direction to achieve a synthetic aperture. The disclosed system also results in a substantial reduction in frame rate.
The reductions in frame rate discussed above are inevitable in the disclosed systems, and in many cases the frame rate may fall to clinically unacceptable levels. Additionally, the multiple firing techniques discussed above reduce frame rate in discrete steps. For example, when two transmit beam firings are required for each transmit beam direction, the frame rate is reduced by a factor of two as compared to conventional single transmit beam operation. It would be advantageous to have a technique whereby a continuous trade off could be made between selected performance factors and the resulting frame rate when employing various multiple-pulse modes of operation.